Much of current research in metal hydride (MH) electrodes follows the principle that the materials should be composed of hydrogen-absorbing elements (A) and non-hydrogen-absorbing elements (B). This classical design principle, however, severely limits the potential capacities of MH electrodes (typically <400 mAh g-1). Herein, we demonstrate a fundamentally new strategy to design high capacity MH electrodes without using B elements by inducing synergism in dehydrogenation process. The Mg24Y5 thin films, which are composed of two strong hydrogen-absorbing elements, achieve an exceptional electrochemical hydrogen storage capacity of ~1500 mAh g-1 (5.6 wt%). All the absorbed hydrogen, including extremely stable H in YH2, can be reversibly desorbed through thermodynamic synergism. The present study provide enlightening insights to design high capacity MH electrodes and therefore enable the revival of Ni-MH batteries, and the emergence of next generation MH-air batteries.